In general, a liquid crystal display (LCD) controls a light transmittance of each liquid crystal cell according to a video signal to display a picture. In other words, a liquid crystal displays contains a plurality of picture elements, or pixels, formed by liquid crystal cells that change the polarization direction of light in response to an electrical voltage of the video signal. By controlling a voltage applied to a liquid crystal cell, the amount of light coming out of the LCD changes. Among various driving methods, active matrix liquid crystal displays, which have a respective switching element such as a thin film transistor for each of the pixels so as to control a voltage to be applied to the liquid crystal, are superior in display quality. Thus, active matrix LCDs have been intensively developed and have come to be widely used as monitors in personal computers.
FIG. 1 shows a perspective view of a conventional LCD which comprises an upper panel 110, a lower panel 120, and liquid crystal materials 130 inserted therebetween. The upper panel 110 contains an upper substrate 112, an upper polarization plate 114, a color filter 116, and a common electrode 118. The lower panel includes a lower substrate 122 and a lower polarization plate 124. The layout of the lower substrate 122 includes a plurality of scanning lines 140, a plurality of data lines 142 which perpendicularly cross the scanning lines, a plurality of thin film transistors 144 (TFTs), and a plurality of pixel electrodes 146.
In FIG. 2, a data driving circuit 210 receives video data signals 212 and polarity control signals 214 and applies pixel data signals to data lines D1-DN. The pixel data signals represent the gray level of red, green, and blue pixels. A scan driving circuit 220 receives scanning control signals 222 and is electrically connected to scanning lines S1-SN. When a voltage is applied to a scanning line, all the TFTs connected to the scanning line are turned on. As a result, the pixel data signals are sent to the pixel electrodes for that scanning line through the TFTs and a voltage is applied to pixel electrodes. On the other hand, a constant voltage Vcom is applied to the common electrode. The difference of voltages between the common electrode Vcom and the pixel electrode creates an electric field resulting in the rotation of liquid crystal molecules and a specific gray level.
Typically, a pixel data signal has either positive polarity or negative polarity depending on whether the voltage of the pixel data signal is higher or lower than a common electrode voltage Vcom. A pixel data signal has positive polarity when its voltage level is higher than the common electrode voltage Vcom. Also, a pixel data signal has negative polarity when its voltage is lower than the common electrode voltage Vcom. The light transmission from the liquid crystal materials (and, therefore, the gray level presented by a pixel,) is related to the difference between the voltages of the pixel data signal and the common electrode voltage Vcom, regardless of the polarity of the pixel data signal. However, a pixel data signal having positive polarity causes liquid crystal molecules to turn to a direction opposite to that caused by a pixel data signal having negative polarity. In order to prolong the lifetime of an LCD, some conventional driving methods such as dot inversion, line inversion, and column inversion are designed to change the polarity of pixel data signals.
FIGS. 3A and 3B are tables showing the polarity of pixel data signals driven by the line inversion method, in which the polarity of pixel data signals is reversed at every scanning line (row). In the column inversion method as shown in FIGS. 4A and 4B, the polarity of pixel data signals is reversed at every data line (column). In the dot inversion as shown in FIGS. 5A and 5B, the polarity is reversed at every row and column. Also, FIGS. 3A, 4A, and 5A represent the polarity status at a specific time frame and FIGS. 3B, 4B, and 5B represent the polarity status at the next time frame. Thus, for any given pixel, the polarity changes each time the pixel is scanned.
At a specific time frame, different polarities of pixel data signals for two adjacent pixels may cause light leakage because of the edge electric field effect resulting from either one of the adjacent pixel electrodes. FIG. 6A shows two adjacent pixels with pixel electrodes 632 and 634, and the data line 625. FIG. 6B is a schematic drawing of a cross-sectional view taken along the section line 6B-6B of FIG. 6A. A TFT layer 620 with a data line 625 is disposed on a substrate 610. The pixel electrodes 632 and 634 are disposed on the TFT layer 620. The liquid crystal material 630 is filled underneath a common electrode 640. A color filter 650 is disposed on the common electrode 640. An edge electric field is generated to effect the rotation of liquid crystal molecules because the polarity of pixel electrode 632 is different from that of pixel electrode 634. As a result, light leakage 660 may occur if the width of data line 625 is not large enough to block the light. If wider data lines are used to prevent light leakage, the aperture ratio of the LCD is sacrificed.
The dot inversion driving method has the serious disadvantage of lower aperture ratio or light leakage. The line inversion driving method has a high system load, because the total voltage level of pixel electrodes connected to a scanning line is high. The column inversion method has the same disadvantage as the dot inversion driving method. Thus, a driving method to resolve these difficulties is desired.